In preparing linear polyesters such as polyethylene terephthalate, a prepolymer consisting of bis(2-hydroxyethyl) terephthalate and/or higher ethylene terephthalate oligomers is polycondensed under vacuum to remove glycol liberated as prepolymer is condensed. The bis-(hydroxy-ethyl) terephthalate is usually prepared by either transesterification of dimethyl terephthalate with ethylene glycol or by direct esterification of highly purified terephthalic acid with excess ethylene glycol at the normal boiling point of the glycol.
U.S. Pat. No. 3,496,220 describes an esterification process in which hydroxyl-ended oligomer, said to be suitable for polycondensation to high molecular weight linear polyesters, is prepared by reacting terephthalic acid with performed hydroxyl-ended oligomer to from a carboxyl-ended oligomer which is then reacted with glycol in successive reaction zones to obtain the hydroxyl-ended oligomer. While it is stated that succinic, pimelic, adipic cyclohexane dicarboxylic, diphenyl dicarboxylic, and isophthalic acid might be used, this esterification process is illustrated only by means of embodiments utilizing the reaction of terephthalic acid with ethylene glycol and formation of the resulting hydroxyl-ended oligomer. No polycondensation of any oligomer is reported.
U.K. Patent Specification No. 1,445,034 describes a process for continuous production of polyester from purified terephthalic acid which polyester is said to have a molecular weight range of 17,500 to 18,500. In this process a slurry of terephthalic acid and excess of ethylene glycol (1.4 mole ratio) along with a conventional esterification catalyst is to be fed into a heated first reactor continuously. Introduction of excess glycol later in the esterification by additional ethylene glycol injection into process streams between successive reactors is said to facilitate completion of esterification.
J. S. Chung in "Acid-Base and Catalytic Properties of Metal Compounds in the Preparation of Poly(ethylene terephthalate)" J. Macromol. Sci.-Chem., A27(4), pp. 479-490 (1990), describes catalytic activity of metal complexes for both transesterification of dimethyl terephthalate and polycondensation of bis(2-hydroxyethyl) terephthalate. Chung reports that some metal ions showing higher activity in the polycondensation reaction also catalyze side reactions which can cause discoloration of the final poly(ethylene terephthalate) polymer. For many commercial uses, such as films and fibers, color is, however, a critical property of poly(ethylene terephthalate) resins.
Preparation of high molecular weight poly(ethylene terephthalate) by means of procedures used for the preparation of polyethylene terephthalate by either transesterification of dimethyl terephthalate with ethylene glycol or by direct esterification of highly purified terephthalic acid fail to produce resins with thermal properties required in applications of commercial interest.
Preparation of poly(ethylene-2,6-naphthalate) from 2,6-naphthalene dicarboxylic acid has, for example, been plagued with problems. These problems are, to a great extent, due to the insolubility of 2,6-naphthalene dicarboxylic acid, the small particle size of 2,6-naphthalene dicarboxylic acid generally available, and a molecular weight of 2,6-naphthalene dicarboxylic acid which is greater than the molecular weight of terephthalic acid. These factors serve to limit the minimum mole ratio of ethylene glycol to 2,6-naphthalene dicarboxylic acid ratio to values greater than two. Under these conditions substantial oligomerization of ethylene glycol occurs in the esterification stage of the preparation. Oligomerization results in a diglycol ether by-product which becomes incorporated into the polyester chain lowering its melting point and generally adversely affecting other properties of the resultant polymer. There is, therefore, a need for and improved process for manufacture from 2,6-naphthalene dicarboxylic acid of high quality polyester resin exhibiting improved thermal properties.